Solid phosphites, particular high performance solid phosphites, are very good stabilizers for the processing of linear low density polyethylene (“LLDPE”), high density polyethylene (“HDPE”), low density polyethylene (“LDPE”) and polypropylene (“PP”). One particular high performance solid phosphite, namely Doverphos® S-9228™ (bis(2,4-dicumylphenyl)pentaerythritol diphosphite) has proven to be a very good polymer stabilizer. (See U.S. Pat. Nos. 5,364,895; 5,438,086; 7,176,252; 6,613,823; 6,224,791; 6,770,693; and 6,680,351.) Although Doverphos® S-9228™ gives excellent color and melt flow stability during processing of LLDPE and LDPE, when used at moderate to high levels (over 250 ppm) it will plate-out on the processing equipment or plate-out on the plastic part after aging, both of which are undesirable.
The problem with plating-out on process equipment is that the there is a build up of residual phosphite on some of the processing parts thereby requiring that the process be stopped and the equipment cleaned. In the case of extruded LLDPE film or blown LLDPE film, the Doverphos® S-9228™ plates out on the rolls resulting in particles of S-9228™ being transferred to the film. Additionally, not only does the S-9228™ plate out on the processing equipment, but on aging some of the S-9228™ migrates to the surface of the polyethylene (“PE”) film. In the case of PE stretch film the S-9228™ appears as a powder on the surface and prevents the stretch film from adhering to itself.
At least one prior art solution described in WO 2006/083642 A1, disclosed a method of reducing the plate-out by the incorporation of a dibenzylidene, with a clarifying agent during polypropylene processing by including the use of one or more co-additives. The co-additives used were selected from the group consisting of homopolymers of ethylene oxide, copolymers containing ethylene oxide segments, propylene oxide and polycaprolactone and derivatives and combinations.
Thus, there is a great need to find a way or process to be able to use solid diphosphites and monophosphites at higher levels in LLDPE and LDPE without the attendant problem of plate-out and/or bloom.